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Project Focus: Off-site construction for pump station replacement

When a Severn Trent pumping station in Derbyshire needed replacement, off-site prefabrication was preferred to overcome the challenge of restricted space

The Weholite wet well was constructed off-siteThe Weholite wet well was constructed off-site

by Frank Thompson, Programme Delivery Leader (East), Severn Trent

The sewage pumping station at The Heage, Sotshole is a relatively small cog in the Severn Trent network, serving around 330 customers living between the village of Heage and the town of Belper, in Derbyshire.

The existing station consisted of a wet and dry well topped by a brick superstructure and had an emergency overflow chamber located within the compound, which was made up of a separate storm well with an overflow to the nearby brook.

The problem that we, as a company, had was that the existing pumping station offered less than two hours of storage at three times dry weather flow and, should pump failure occur, the surcharging system would result in pollution incidents to the brook.

Alongside this fundamental issue, the existing mechanical and electrical equipment was deteriorating and was no longer compliant with our own internal standards, further increasing the risk of pollution due to pump failure and blockage. In addition there was a clear risk to our own personnel due to the general decline of the pump station. We therefore received approval to replace the pumping station as a matter of priority. However, it was not a straightforward job for ourselves and for our lead design and build contractor, NMCNomenca.

We faced a number of issues:

• The existing wet and dry wells had to be abandoned;
• Existing mechanical and electrical assets had to be decommissioned to remove any potential hazards;
• We were working in a compound that was next to an area of ecological importance; and
• The compound itself had limited storage for materials and waste.

The last of those problems, the restricted nature of the compound, led to the project team proposing an innovative design that would result in an entirely prefabricated solution that could be lifted into place piece by piece over the course of just 11 days. It would be manufactured off-site, with an extended design phase ensuring reduced health and safety risks and improved environmental credentials over the construction and future operation of the asset, together with the 20% improvement in efficiency we called for as part of our AMP 6 plans.

The new design called for a 3.5m diameter, 4.5m deep chamber to allow for the required pump station storage, while two new 7.5kW submersible pump units would deliver the necessary flows downstream. In addition, the refurbishment of the existing emergency overflow chamber included a new static bar screen that would also serve to provide additional resilience against potential future pollution events.

In order to achieve the build within a restricted compound, and also taking into account the adjacent site of ecological importance, the team decided that the only way to achieve all of that was to go with off-site prefabrication, with only final assembly and connections taking place on site. That meant the concrete valve chamber and the Weholite wet well were both constructed entirely off site and then shipped to the compound to be ‘slotted’ into place. In fact, the Weholite wet well chamber came complete with fittings, the precast concrete cover slab arrived complete with pump access covers, and the precast valve chamber was fully fitted with valves, pipework, flowmeters, access ladder and open mesh flooring.

The designers also had ease of long-term maintenance at the forefront of their minds, with the inclusion of new covers providing the safest possible access arrangement, and a new lifting davit system. A new upstream inlet manhole to isolate and divert flows was also included. Severn Trent then also modified the existing control panels to include level instrumentation and flowmeter telemetry to provide us with real-time operational information which is used to monitor and review the station’s performance.

The innovative nature of the design meant that on-site construction took just 11 days to be completed. The general timetable was excavation on days one to four, with the valve chamber installed first, followed by the Weholite chamber on day five. The tank was then filled with water after installation to prevent uplift and to aid structural stability during concreting. Days six to 10 saw five concrete pours (one on each day) to produce a 1m surround that was allowed to cure over the weekend. The final activity, on day 11, was to set the pre-cast cover slabs in place and to install guide rails and pumps, and to finish off the duct work.

In total, we estimate that, by taking this approach, we saved two to three weeks in comparison to other potential, traditional construction routes. Factory thinking approaches led to a safer assembly, which helped reduce the overall health and safety and environmental risk from on-site construction, and the quality controlled prefabricated environment also helped guarantee consistent products. Being able to cut the construction period to just 11 days on site also saw reductions in stakeholder and customer disruption, as well as ensuring a reduction in accompanying costs.

Alongside the operational benefits associated with the new pumping station, the decision to construct using prefabricated section also helped with the project’s environmental benefits. Site waste was reduced by almost three-quarters when compared with similar sized schemes, with 100% of the waste recycled off site.

There was also a significantly reduced carbon footprint in comparison to a more traditional concrete pumping station solution thanks to the reduction in length of the overall construction programme and the use of recycled materials in the manufacture of the Weholite wet well. The 120-year design life of the wet well should also ensure that all future site work is limited to simple maintenance and minor modifications.

While the decision to use an off-site prefabricated design was necessitated by the nature of the site at Heage, it was clear by the end of the project that, in terms of quality control, reduction in construction timescales and improvement in environmental impact, there are potentially huge benefits in using this method for similar sized projects that are less constrained.

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